Oxygen Reduction Reaction at Single Entity Multiwalled Carbon Nanotubes

The electrocatalysis of the oxygen reduction reaction (ORR) in aqueous base (0.1 M KOH) by multiwalled carbon nanotubes (MWCNTs) is studied at the single entity level. Electroactive surface functionality is shown to facilitate significant electrocatalysis leading to peroxide formation which is seen to occur at lower potentials as compared to the voltammetric responses obtained at bare carbon macroelectrodes and at such electrodes modified with layers of carbon nanotubes.


Section 1: Experimental Section
Chemicals and reagents.
All chemicals were of analytical grade and were used as received without any further purification.
Bamboo like multiwall carbon nanotubes (MWCNTs, BPD30L5-20) were purchased from Nanolab ((Brighton, MA, USA). Transmission electron microscopy (TEM) of the sample of MWCNTs is shown in Figure S1 supplied by NanoLab 1 . XPS data analysis 2 of MWCNTs indicated that the sample was 98.9 atomic% carbon, 1.0 atomic% oxygen, 0.1 atomic% iron, and trace (< 0.1 atomic%) copper and sulfur was observed. The dimensions for carbon nanotubes are listed in Table S1.  Number of concentric tubes 9 Potassium Hydroxide (KOH, 85%) and Ethanol (≥99.8% purity) were obtained from Sigma-Aldrich (Dorest, UK). All solutions were prepared in ultra-pure water at a resistivity of 18.2 MΩ cm at 298K (Millipore, MA, USA) and were deaerated thoroughly with nitrogen (99.998%, BOC Gases plc) before use.

Electrochemical instrumentation and methods
All electrochemical experiments were conducted in a Faraday cage at 298 ± 0.5 K using an EC-Lab potentiostat (SP-200 with an ultra-low current module, Biologic Science Instruments, France).

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For cyclic voltammetry (CV), a glassy carbon macroelectrode (GCE, diameter = 3.02 +/-0.005mm) was used as the working electrode, a Ag/AgCl electrode (in 3.5 M KCl solution) used as the reference electrode and a graphite rod as the counter electrode. The GCE was polished carefully using alumina with decreasing particle sizes of 1.0, 0.3 and 0.05 μm (Buehler, IL, UK) in turn on soft lapping pads (Buehler, UK), followed by rinsing with ultra-pure water and drying with nitrogen.
For nano-impact experiment, the chronoamperometry was conducted using a carbon fibre micro-wire electrode of 7 µm diameter and 1 mm in length as the working electrode, the same reference electrode and counter electrode were used as above. The fabrication method of carbon fibre micro-wire electrodes has been reported previously 3 .

Cyclic voltammetry of oxygen reduction reaction (ORR)
A suspension of 0.05 g L -1 MWCNTs was prepared by adding 0.5 mg of MWCNTs to 10 mL ethanol.

Nano-impact experiments of MWCNTs
The measurements of current for ORR on single carbon nanotube were based on the general method MWCNTs were added. The data analysis program "Signal Counter" (Centre for Marine and Environmental Research, Zagreb, Croatia) 5 and OriginPro 2020 were employed for impact signal identification and analysis.

Section 2: Estimation of the number of layers of MWCNTs drop-casted on the GC electrodes
To estimate the minimum number of MWCNT monolayers on the bare GC electrode, we assume a closed pack arrangement of the MWCNT is laid uniformly across the whole GC surface. This gives a lower limit estimate for the number of layers.
The diameter and length of the bamboo-like multiwalled carbon nanotubes (purchased from NanoLab, USA) is ca. 30 nm and ca. 20 µm respectively.
Assuming the MWCNTs contact at the GC surface to be rectangular when arranged in a closed pack manner, The area covered by one MWCNT: The modification amount of MWCNTs (2 μL 0.05 gL -1 ) on the bare GC electrode: The number of MWCNTs modified on the surface of modified GCE N MWCNT =1×10 -7 g / 1.3×10 -14 g = 7.7×10 6 (the mass per carbon tube is 1.3×10 -14 g 6 ) Hence, the total area covered by 7.7×10 6 MWCNTs S total =6 ×10 -9 cm 2 × 7. All cyclic voltammograms of MWCNTs/GCE recorded in the absence of oxygen in 0.1 M KOH were baseline subtracted to obtain the reported values of reductive peak currents as illustrated in Figure S2.
The background correction for the raw cyclic voltammograms were conducted by using OriginPro 2020, a baseline was fitted between the potentials at which the surface bound voltammetric wave was judged to have started and ended, then subtracted from the peak. CVs were recorded at the MWCNTs/GCE in degassed 0.1 M KOH by scanning first from 0 V to 0.5 V and holding the potential at 0.5 V before making the cathodic scan. Under these conditions the reductive peak appearing at -0.35 V decreased in size compared with the CV recorded directly from 0 V to -1 V ( Figure S4(A)). Furthermore, as the holding time at + 0.5 V was extended to 1, 3 and 5 mins, the reductive S6 peak gradually decayed until disappeared ( Figure S4(B)). It was inferred that the surface oxygen groups on MWCNTs may be irreversibly converted to carboxylic or other electro-inactive groups under potential control at + 0.5V.  The number of hexagons (N h ) on the external surface of a single MWCNTs particle is N h = S mw / S h =1.9 ×10 -12 m 2 / 5.2 × 10 -20 m 2 = 3.6 × 10 7 The amount ratio between quinone groups and hexagons is N q / N h = 1:6.

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Section 5: Nano-impact experiment of MWCNTs in the absence of oxygen Single entity measurements were also conducted at oxidizing potentials. Extending the applied potential positive than 0.2 V, oxidative impacts appeared as shown in Figure S6(A) and demonstrated a potential dependency for the impact current height and charge as shown in Figure S6(B). At the more positive potentials the impacts with an average frequency of 0.066 ± 0.015 s -1 ( Figure S8(B)) show a slightly longer average impact time of length 42 ± 13 ms (see Figure S7

Section 6: Voltammetry at unmodified carbon electrodes in the presence of oxygen
To investigate the ORR performed on a bare GCE in 0.1 M KOH, CVs were scanned for scan rates varying from 10 mVs -1 to 200 mVs -1 on a bare GCE first from 0 V to -1.4 V vs Ag/AgCl respectively, as shown in Figure S9(A). The inset shows dependence of the peak current on the square root of voltage scan rate hinting at probable diffusion control and the formation of peroxide via a two-electron process as outlined in reference 7 . The Tafel slope is found to vary as a function of scan rate and the calculated cathodic transfer coefficients α c decreases from 0.79 to 0.48 as scan rate increases ( Figure S9(B)) according to Equation S1 defined by the International Union of Pure and Applied Chemistry (IUPAC) 8 as follows: Eq.(S1) where I c is the experimentally measured cathodic current (in the range of 20 % -30 %), E is the potential of the working electrode, F is the Faraday constant (96485 C mol -1 ), R is the Gas Constant (8.314 J mol -1 K -1 ) and T is the experimental temperature (298 K). The interpretation of these data has been given elsewhere 7 where the formation of peroxide species is confirmed and the role of surface adsorbed species is emphasized.

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Section 12: Calculation of the transport limited current for single carbon nanotubes To interpret the single nano-imapct events we approximately view the impacted MWCNT as a cylindrical electrode with uniform diffusional access to its surface to which the transport limited current is given by Szabo et al. 9 :